The present invention is related to ion-sensitive electrodes which provide an electrically measurable sensor for determining ion concentration in fluids.
Generally, ion-sensitive electrodes depend upon a particular ion in a fluid interacting with a membrane to modify the electrical characteristics of a measuring electrode. In recent years, the ion-sensitive field effect transistor (ISFET) has been drawing increasing interest from the scientific and technological community. One example of the ISFET is taught in U.S. Pat. No. 3,831,432 and herein incorporated by reference. These have numerous advantages stemming from their small size and the fact that they are truly semiconductor devices.
However, there are difficulties with the field effect transistor in that the source-drain biasing potential, no matter how small, will polarize the ionic solution in the vicinity of the source-drain electrodes. The effect of this polarization by the unscreened field is considered by many to produce slow drifts in the output which impairs the long-term stability in the field effect transistor. If an alternating current signal is used, to prevent slow drifts, the large Miller capacitance of the overlap region of the channel in the Field Effect Transistor will cause shunting of the signal through the ionic solution. The least effect of this shunting through the ionic solution is to change the response to the ion concentration in the vicinity of the ion-sensitive membrane resulting in inaccurate indication of the ion concentration in the fluid.
Additionally, the ISFET is not suitable for multiple ion sensing due to the complexity of connections which must be made to the ISFET associated with each different ion sensor. This complexity results in making the incorporation of the ISFET into integrated circuits more difficult and expensive.
Accordingly, an object of the present invention is to provide an ion-controlled diode (ICD) in order to measure, without adversely affecting, the ion concentration of fluids.
A further object of the present invention is to provide an ion-controlled diode which is capable of providing an analog or a digital output indicative of ion concentration, rendering it more compatible with integrated circuit technology.
It is a further object of the present invention to eliminate the polarization of an ionic solution, whose ion concentration is to be measured, by the measuring device itself.
A still further object of the present invention is to provide a high degree of isolation between the ion sensor region and the electronic sensing circuitry in an ion measurement device.